1. Field of Invention
The present invention is directed to the hydrogenation of polycrystalline silicon devices, such as thin-film transistors. More specifically, the present invention relates to a method for the enhancement of thin-film transistor hydrogenation using a metal capping layer. The present invention is also directed to a method for the batch hydrogenation of polycrystalline silicon thin-film transistors.
2. Description of Related Art
Polycrystalline silicon ("polysi") thin-film transistors ("TFT") are made on large glass plates for use in numerous applications, including, but not limited to, flat panel displays, scanners, printers, imagers, and the like.
The dangling bonds present in the polycrystalline silicon grain boundaries and at intra-grain defects need to be passivated by bonding them to hydrogen atoms in order to improve electrical performance of the thin-film transistors. This is usually done after the thin-film transistors have been patterned and passivated with an oxide coating. Otherwise, high-temperature processing (&gt;350.degree. C.) will drive the hydrogen out from the silicon.
Polycrystalline silicon thin-film transistors are presently passivated with hydrogen atoms by exposing the thin-film transistor plates to a hydrogen gas plasma. The hydrogen plasma contains variously active forms of hydrogen, mainly positive ions (H.sup.30) and hydrogen atoms (H). Under these conditions, a long exposure time of up to 16 hours is required to achieve full hydrogenation and saturation of the thin-film transistors.
Because the hydrogenation time is long and each thin-film transistor plate must be exposed to the hydrogen plasma one plate at a time, throughput is low and cost is high.
It is known in the art that the hydrogen passivation time of a thin-film transistor can be shortened by utilizing a non-doped insulating silicon oxide layer having a thickness less than 100 nm on top of a polycrystalline silicon transistor, as described in U.S. Pat. No. 5,470,763 to Hamada, the entire disclosure of which is herein incorporated by reference. However, the hydrogenation time for passivation reported in Hamada is 2.5 hours and contamination from an overlying boron-included phosphorus silicated glass (BPSG) layer is a concern.
Thus, there is a need to greatly improve hydrogenation efficiency, thereby decreasing the time for hydrogenation as well as the cost of the equipment, and allowing a stack of thin-film transistor plates to be batch hydrogenated for increased throughput.